1. Field of the Invention
The present invention relates to a catalyst carrier made of a stainless steel ribbon, used for the exhaust-gas purification device of an automobile, and a method for producing the same.
2. Description of the Related Arts
Recently, particularly from about 1970, control of the exhaust gas of automobiles has been seriously considered. Accordingly, a law concerning the control of the automobile exhaust gas emissions was promulgated to the effect that the passenger automobiles must be equipped with a device for purifying the exhaust gas. Several systems were proposed for devices for purifying the exhaust gas, but the most widely used at present is a catalyst converter system, according to which HC and CO are oxidized and, simultaneously, NOx is reduced. The structure of these catalyst converters is such that a carrier, which is a honeycomb sintered body of ceramic mainly composed of corderite, is mounted in a metallic cylinder, as a fundamental body, and porous .gamma.-Al.sub.2 O.sub.3 powder, which is impregnated with a noble metal (Pt.about.Pd) catalyst, is disposited on the carrier. The honeycomb ceramic has, however, disadvantages in that it is not resistant to mechanical impact, a part of the exhaust gas is able to bypass the conversion by escaping through the clearance between the honeycomb and the outer cylinder, and in addition, the exhaust resistance is rather high. To cope with these disadvantages, Japanese Unexamined Patent Publication No. 57-71898 disclosed a several tens of microns thick ribbon rolled from high Al ferritic stainless steel laminated with a corrugated sheet which is produced by forming the ribbon, or coiled in the form of a roll, thereby enabling it to be used as the carrier instead of the ceramic honeycomb mentioned above. Japanese Unexamined Patent Publication No. 56-96726 also disclosed a round bar of high Al ferritic stainless steel which is subjected to peeling to obtain cutting chips for producing a ribbon used for the metal honeycomb. Since a high degree of working strain during the peeling is introduced into the stainless steel ribbon produced by this method, when the ribbon is treated in a high temperature, oxidizing atmosphere, the diffusion of Al from the interior to the surface of the ribbon is promoted, which results in the formation of Al.sub.2 O.sub.3 in the form of whiskers, which enhances a rigid bonding of .gamma.-Al.sub.2 O.sub.3 powder, i.e., the direct carrier of noble metal-catalyst, on the honeycomb. Nevertheless, these techniques have several disadvantages, in that, in the technique for producing a ribbon for the carrier by a rolling method, since a high Al stainless steel, whose workability is inherently not excellent, is cold-rolled to obtain an approximately 40.about.50 .mu.m thick ribbon, the annealing and pickling steps must be repeated several times during the course of the rolling, which leads to an increase of costs. In addition, several limitations must be interposed on the alloying components from the viewpoint of rollability, with result that a satisfactory oxidation resistance property is obtained only with difficulty. That is, since Al in the ribbon is consumed, while forming a selective oxidation film made of Al.sub.2 O.sub.3 during service at a high temperature and a thickness of a ribbon used for the catalyst carrier of only tens of microns, the Al concentration of a ribbon decreases during service. Accordingly, Al in the ribbon may be consumed prior to the formation of a satisfactorily thick and protective Al.sub.2 O.sub.3 film at a service temperature. In this case, Cr and Fe are then oxidized with an increased speed, and finally, the entire ribbon is oxidized. Accordingly, to enhance the durable service temperature, the Al content in a ribbon must be as high as possible. On the other hand, since Al in a high concentration degrades the rollability, the Al content is limited to approximately 5% at maximum as long as the ribbon is produced by the rolling method. This Al content limits the durable service temperature in exhaust gas to 950.degree. C. at the highest, when the thickness of a ribbon is 40 .mu.m. It seems obvious that this durable limit is not satisfactory for the various conditions anticipated.
Also for the peeling technique, it is difficult to stably obtain a ribbon having a uniform thickness and width. Investigation, therefore had been made into the development of a catalyst carrier for an exhaust gas of an automobile, consisting of a stainless steel ribbon with a higher Al content, and thus an improved oxidation-resistance, and having an improved adhesion of .gamma.-Al.sub.2 O.sub.3 powder, i.e., the direct carrier of the catalyst, and a method for producing such a catalyst carrier.